The present invention relates primarily to the acquisition of subsurface acoustic data with a borehole geophone cable and in one of its preferred aspects relates to a receiver system especially useful in seismic tomography. The cable has additional facilities for recording detailed subsurface pressure and temperature data which is required to interpret the acoustic data.
Cross-borehole seismic tomography is currently being used as a method for subsurface characterization and monitoring changes within subterranean formations. For example, seismic tomographic imaging has been used to monitor the movement of thermal fronts through reservoirs in enhanced oil recovery projects, and the results from these surveys indicate that the tomographically-processed, cross-borehole data can image the velocity of the reservoir. Comparison of time sequential images can then be used to determine changes in the physical properties of the formation as a function of time and the recovery processes. Tomographic data is acquired by placing a seismic source in a "source" borehole and a seismic receiver in a "receiver" borehole, both of which penetrate the section of the formation to be imaged. The energy source is "fired" to generate acoustical energy which propagates through the formation and is recorded at the receiver borehole. The time required for the energy to propagate between the boreholes is dependent on the path along which the energy traveled. By positioning the source and receivers at different depths in their respective boreholes, a large number of different "travel paths" can be sampled and recorded. This information can be processed into a cross-sectional tomographic image of the section of the formation bounded by the two boreholes.
Typical receiver systems have been comprised of a plurality of three-component geophones located at spaced-apart positions along a borehole cable, either suspended in the borehole or installed directly in the subsurface. While numerous configurations of borehole cables have been employed in carrying out such tomographic surveys in moderate environments, it is the specific object of the present invention to provide a borehole cable that is capable of operating at pressures greater than 10,000 psi and temperatures greater than 450.degree. F. in permanent installations.